1. Field of the Invention
The invention disclosed broadly relates to phase shifting, and more particularly relates to constant-attenuation continuously variable phase shifters and methods of optimizing their performance.
2. Description of the Related Art
Typical phase shifters comprise a digital input and the device is capable of a fixed number of discrete phase delays. An 8-bit device can therefore produce 256 different discrete phase delays by means of switching on and off, as required, transmission lines of fixed lengths. Assuming that these phase delays are evenly distributed throughout 180 degrees, as is common, then the phase shifter would have a resolution of 0.7 degrees (180 degrees/256). Even if a device with a greater number of bits is used, it is always possible that a circuit designer will need a phase shifter with a greater, or even continuous, resolution, either for smaller desired changes in phase or for achieving certain specific values of phase delay.
Another disadvantage of typical phase shifters, and therefore of typical phase shifter and amplitude controller pairs ("phase/amplitude controllers"), is that neither variable is independent of the other. Changes in phase produce changes in amplitude (attenuation), and changes in amplitude also produce changes in phase. The fact that both variables are changing at the same time makes it difficult to set the system or circuit to particular values of phase and amplitude, and it also makes the optimization of a system or circuit more complex.
The optimization of a typical phase/amplitude controller can be a lengthy process. It can require the search of every phase and amplitude combination, changing one variable at a time, and the comparison of the results for each of these combinations. Further, even if the circuit is initially optimized, a change in environmental conditions like temperature, pressure, humidity, etc., will move the optimization point. This causes a deterioration in the performance of the circuit unless, or until, the phase/amplitude controller is reoptimized. Such reoptimization can be difficult and can itself cause a deterioration in performance because the new optimization point for the phase/amplitude controller cannot be searched for off-line. The phase/amplitude controller must actually be set to each of the different phase and amplitude values (many of which produce poor performance) on-line without noticeably disturbing the operation of the circuit, and the performance of the circuit for each set of values must be measured.
Accordingly, there is a need for a phase shifter and a phase/amplitude controller, and methods of initially optimizing and subsequently updating them, which overcome these problems.